1. Field of the Invention
The present invention relates to a zoom lens, and, more particularly, to a zoom lens which is suited for a television camera, a photographic camera, a digital camera, a video camera or the like, and in which an aspheric surface is appropriately employed in a part of a lens system thereof to obtain good optical performance over the entire variable magnification range while having, for example, a large relative aperture of 1.5 or thereabout in F-number at the wide-angle end, an ultrawide angle of view (2xcfx89(angle of view in the wide-angle end)=78xc2x0-95xc2x0) and a high variable magnification ratio of 10-27 or thereabout. Further, the present invention relates to a zoom lens which is suited for a television camera, a video camera, a photographic camera, a video camera or the like, and, more particularly, to a zoom lens, in which the so-called inner focusing method, in which focusing is effected by moving a lens subunit that is a part of a first lens unit, is employed to obtain high optical performance over the entire object distance range while having a short minimum object distance.
2. Description of Related Art
Heretofore, for use with a television camera, a photographic camera, a digital camera, a video camera or the like, there has been a demand for a zoom lens having high optical performance while having a large relative aperture and a high variable magnification ratio.
In addition to such a demand, in the case of a color television camera for broadcasting, in particular, importance is attached to operability and mobility. In response to such a requirement, the usage of a CCD (charge-coupled device) of ⅔ inch or xc2xd inch has become the mainstream for an image pickup device in the color television camera.
Since the CCD has an almost uniform resolution over the entire image pickup range, a zoom lens to be associated with the CCD is also required to have an almost uniform resolution from the center of an image plane to the periphery thereof.
For example, the zoom lens is required to have the various aberrations such as astigmatism, distortion and lateral chromatic aberration corrected well and to have high optical performance over the entire image plane. In addition, the zoom lens is required to have a large relative aperture, a wide angle of view and a high variable magnification ratio while being small in size and light in weight, and moreover to have a long back focal distance for enabling a color separation optical system and a variety of filters to be disposed in front of an image pickup means.
Among zoom lenses, the so-called four-unit zoom lens, which is composed of four lens units, i.e., in order from the object side, a first lens unit of positive refractive power for focusing, a second lens unit of negative refractive power for variation of magnification, a third lens unit of positive or negative refractive power for compensating for shift of an image plane caused by the variation of magnification, and a fourth lens unit of positive refractive power for image formation, is relatively easy to make have a high variable magnification ratio and a large relative aperture and is, therefore, widely used as a zoom lens for color television cameras for broadcasting.
Among the four-unit zoom lenses, a zoom lens having a large relative aperture and a high variable magnification ratio, such as having an F-number of 1.7 or thereabout, an angle of view at the wide-angle end 2xcfx89 of 86xc2x0 or thereabout, and a variable magnification ratio of 8 or thereabout, has been proposed, for example, in Japanese Laid-Open Patent Application No. Hei 6-242378.
In order to obtain, in a zoom lens, a large relative aperture (F-number of 1.5-1.8), a high variable magnification ratio (variable magnification ratio of 10-27) and an ultra-wide angle of view (angle of view in the wide-angle end 2xcfx89 of 78xc2x0-95xc2x0) and, moreover, to have high optical performance over the entire variable magnification range, it is necessary to appropriately set the refractive power of each lens unit and the lens construction.
In general, in order to obtain high optical performance with little variation of aberrations over the entire variable magnification range, it becomes necessary to increase the freedom of design on aberration correction, for example, by increasing the number of lens elements of each lens unit.
Therefore, if it is attempted to attain a zoom lens having a large relative aperture, an ultra-wide angle of view and a high variable magnification ratio, a problem arises in that the number of lens elements would be inevitably increased to make the size of the whole lens system large. Thus, it would become impossible to meet the requirement for reduction in size and weight.
Further, with respect to the image forming performance, first, making reference to the ultra-wide angle of view of a zoom lens, the greatest problem is distortion. This is because distortion has influence according to the cube of an angle of view in a region of third-order aberration coefficients.
FIG. 45 is a schematic diagram showing the variation of distortion in every zoom position.
As shown in FIG. 45, distortion exhibits a considerably large under-tendency (minus tendency) when the zoom position is at a wide-angle end (focal length of fw). As zooming advances from the wide-angle end fw to a telephoto end (focal length of ft), distortion becomes gradually large in the direction of an over-tendency (plus tendency). Then, after zooming reaches a zoom position at which the value of distortion is xe2x80x9c0xe2x80x9d, the value of distortion in the over-tendency becomes maximum when the zoom position is in the vicinity of fm=fwxc3x97Zxc2xc, where fw is a focal length at the wide-angle end and Z As a zoom ratio. After that, as zooming advances from the position of the focal length fm to the telephoto end ft, the value of distortion in the over-tendency becomes gradually small. Such an inclination of distortion becomes larger as an angle of view at the wide-angle end becomes larger. Therefore, in such an ultra-wide-angle zoom lens as to have an angle of view 2xcfx89 at the wide-angle end exceeding 78xc2x0, distortion in the under-tendency increases rapidly on the wide-angle side, so that it becomes very difficult to control distortion.
The next problem is that a point at which an image contrast becomes best in the center of an image plane, i.e., the so-called best image plane, varies due to the variation of magnification. This is mainly caused by the variation of spherical aberration due to the variation of magnification. Since the spherical aberration has influence according to the cube of an aperture in a region of third-order aberration coefficients, it presents the greatest problem for attaining a large relative aperture.
In general, the variation of spherical aberration due to the variation of magnification exhibits, as shown in FIG. 46, an under-tendency (minus tendency) with respect to a Gauss image plane when zooming advances from the wide-angle end at which the value of spherical aberration is xe2x80x9c0xe2x80x9d until the vicinity of the zoom position fm=fwxc3x97Zxc2xc where Z is a zoom ratio and fw is a focal length at the wide-angle end. Then, when zooming passes the vicinity of the zoom position fm=fwxc3x97Zxc2xc, the value of spherical aberration in the under-tendency becomes small. After zooming passes a zoom position at which the value of spherical aberration is xe2x80x9c0xe2x80x9d, spherical aberration comes to exhibit an over-tendency (plus tendency) in turn.
Then, in the vicinity of a zoom position fd=(Fno.w/Fno.t)xc3x97ft at which the so-called F drop begins, i.e., the zoom position where the F-number begins to become large (the lens-system begins to become dark) with the diameter of an on-axial light flux limited, spherical aberration exhibits the greatest over-tendency (plus tendency). When zooming passes the zoom position fd, the value of spherical aberration in the over-tendency becomes small. At the telephoto end, the value of spherical aberration becomes almost xe2x80x9c0xe2x80x9d. Incidentally, Fno.w and Fno.t represent the F-numbers at the wide-angle end and the telephoto end, respectively, and ft is a focal length at the telephoto end.
As mentioned above, in particular, in such a zoom lens as to have a zoom position where the F drop begins, it is very difficult to control spherical aberration on the telephoto side.
In order to correct well the variation of various aberrations over the entire variable magnification range, in the past, the number of constituent lenses of a focusing lens unit or a variator lens unit has been made to increase. Therefore, there has been a problem that the whole of the lens system is increased in size and complicated in construction.
Further, the introduction of an aspheric surface for the purpose of solving the above problem has been proposed in the above-mentioned Japanese Laid-Open Patent Application No. Hei 6-242378.
However, with regard to a zoom lens of improved performance having a large relative aperture and, moreover, a high variable magnification ratio beginning with an ultra-wide angle, it has been becoming necessary to reconsider the method of introducing an aspheric surface.
In a zoom lens having a large relative aperture and, moreover, a high variable magnification ratio beginning with an ultra-wide angle, distortion varies greatly on the wide-angle side and spherical aberration varies greatly on the telephoto side. Even if an aspheric surface is merely introduced to any one of surfaces of a variator lens system so as to correct well both distortion and spherical aberration, it has been becoming difficult to correct such aberrations efficiently and well.
Meanwhile, among zoom lenses each comprising, in order from the object side, a first lens unit (focusing lens unit) of positive refractive power for focusing, a second lens unit (variator lens unit) of negative refractive power for variation of magnification, a third lens unit (compensator lens unit) of positive or negative refractive power for compensating for shift of an image plane caused by the variation of magnification, an aperture stop, and a fourth lens unit (relay lens unit) of positive refractive power for image formation, i.e., among the so-called four-unit zoom lenses, a zoom lens employing the inner focusing method in which focusing is effected by moving a lens that is a part of the first lens unit has been proposed in Japanese Patent Publication No. Sho 59-4686, the above-mentioned Japanese Laid-Open Patent Application No. Hei 6-242378, etc.
In the above zoom lens, the first lens unit is composed of three lens subunits, i.e., a first lens subunit of negative refractive power, a second lens subunit of positive refractive power and a third lens subunit of positive refractive power, and focusing from an infinitely distant object to a minimum-distance object is effected by moving the second lens subunit toward the image side.
In general, a zoom lens employing the inner focusing method, as compared with a zoom lens in which focusing is effected by moving the whole first lens unit, has such advantageous characteristics that the effective diameter of the first lens unit can be made small to easily reduce the size of the whole lens system, close-up photography, in particular, ultra-close-up photography, can be made easy, and rapid focusing can be performed since focusing is effected by moving a relatively small-sized and light-weight lens unit to make the driving force for the lens unit small.
Generally, if the inner focusing method is employed in a zoom lens, as mentioned in the foregoing, there can be obtained such advantageous characteristics that the size of the whole lens system can be reduced, rapid focusing becomes possible, and close-up photography becomes easy.
However, on the other hand, if a zoom lens is made to have a large relative aperture (F-number of 1.6), a high variable magnification ratio (zoom ratio of 8-15 or thereabout) and a wide angle of view while reducing the minimum object distance, as mentioned in the foregoing, such a problem becomes conspicuous that distortion increases, and, in particular, distortion in the minus tendency at the wide-angle end increases.
The reason for such an increase of distortion is that, in order to attain a wide angle of view while reducing the minimum object distance, it is necessary to strengthen the power of a lens subunit (first lens subunit) located on the most object side in the first lens unit, thereby causing distortion in the minus tendency at the wide-angle end to increase.
In general, in order to obtain high optical performance over the entire variable magnification range, it becomes necessary to increase the freedom of design on the correction of aberrations, for example, by increasing the number of lens elements of each lens unit.
Therefore, if it is tried to attain a zoom lens having a large relative aperture, a wide angle of view and a high variable magnification ratio, a problem arises in that the number of lens elements increases to make the size of the whole lens system large, so that it becomes impossible to meet the demand for reduction in size and weight.
The invention has been made in view of the above-mentioned problems, and an object of the invention is to provide a zoom lens having high optical performance over the entire variable magnification range with the variation of various aberrations due to the variation of magnification lessened, in particular, with distortion on the wide-angle side and spherical aberration on the telephoto side corrected well, by, in the so-called four-unit zoom lens, appropriately setting the refractive power of each lens unit, the F-number, etc., and applying aspheric surfaces to at least two lens surfaces.
Another object of the invention is to provide a zoom lens having a large relative aperture and a high variable magnification ratio such as having an F-number of 1.5-1.8 or thereabout, an ultra-wide angle of view (angle of view at the wide-angle end 2xcfx89=78xc2x0-95xc2x0 or thereabout) and a variable magnification ratio of 10-27 or thereabout.
A further object of the invention is to provide a zoom lens employing the inner focusing method in which focusing is effected by moving a lens subunit for focusing that is a part of a first lens unit of the four-unit zoom lens, and having high optical performance with distortion in the minus tendency at the wide-angle end corrected well while attaining a wide angle of view, a reduction in minimum object distance, and a high variable magnification ratio and attaining the reduction in size of the whole lens system.
To attain the above objects, in accordance with an aspect of the invention, there is provided a zoom lens comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power for variation of magnification, a third lens unit for compensating for shift of an image plane caused by the variation of magnification, and a fixed fourth lens unit of positive refractive power, wherein the first lens unit comprises a first lens subunit of negative refractive power fixed during focusing, a second lens subunit having a focusing function, and a third lens subunit of positive refractive power fixed during focusing, and at least one. of the first lens subunit and the second lens subunit and the third lens subunit respectively have aspheric lenses.
In accordance with a preferred aspect of the invention, in the above zoom lens, the first lens subunit and the third lens subunit respectively have aspheric lenses.
In accordance with a preferred aspect of the invention, in the above zoom lens, the first lens subunit has an aspheric surface (AS1) formed on at least one lens surface thereof satisfying the following conditions:
1.30 less than hw/ht, and 1.05 less than hw/hz
where ht is a maximum height of incidence of an on-axial light flux in the first lens subunit, hw is a height of incidence of an off-axial light flux of a maximum angle of view in the first lens subunit at a wide-angle end, and hz is a height of incidence of an off-axial light flux of a maximum angle of view in the first lens subunit at a zoom position corresponding to Zxc2xc, Z being a variable magnification ratio of the zoom lens.
In accordance with a preferred aspect of the invention, in the above zoom lens, the third lens subunit has an aspheric surface (AS2) formed on at least one lens surface thereof satisfying the following conditions:
0.75 greater than hw/ht
where ht is a maximum height of incidence of an on-axial light flux in the third lens subunit, and hw is a height of incidence of an off-axial light flux of a maximum angle of view in the third lens subunit at a wide-angle end.
In accordance with a preferred aspect of the invention, the above zoom lens satisfies the following conditions:
Z greater than 10.
xe2x88x920.42 less than xcex22w less than xe2x88x920.18xe2x80x83xe2x80x83(1)
xe2x80x83xe2x88x922.45 less than f11/f1 less than xe2x88x920.98xe2x80x83xe2x80x83(2)
1.05 less than f13/f1 less than 2.10xe2x80x83xe2x80x83(3)
where Z is a variable magnification ratio of the zoom lens, f1, f11 and f13 are focal lengths of the first lens unit, the first lens subunit and the third lens subunit, respectively, and xcex22w is a lateral magnification at a wide-angle end of the second lens unit.
In accordance with a preferred aspect of the invention, in the above zoom lens, the aspheric surface (AS1) has, when formed on a positive refractive surface, such a shape that a positive refractive power becomes progressively stronger toward a marginal lens portion thereof, or has, when formed on a negative refractive surface, such a shape that a negative refractive power becomes progressively weaker toward a marginal lens portion thereof, and the following conditions are satisfied:
1.07xc3x9710xe2x88x923 less than |xcex941(10)/f1| less than 7.20xc3x9710xe2x88x922
1.06xc3x9710xe2x88x923 less than |xcex941(9)/f1| less than 4.90xc3x9710xe2x88x922
6.10xc3x9710xe2x88x924 less than ⊕xcex941(7)/f1| less than 1.95xc3x9710xe2x88x922xe2x80x83xe2x80x83(4)
where xcex941(10), xcex941(9) and xcex941(7) are amounts of asphericity at portions corresponding to 100%, 90% and 70%, respectively, of an effective lens diameter of the aspheric surface (AS1), and f1 is a focal length of the first lens unit.
In accordance with a preferred aspect of the invention, in the above zoom lens, the aspheric surface (AS2) has, when formed on a positive refractive surface, such a shape that a positive refractive power becomes progressively weaker toward a marginal lens portion thereof, or has, when formed on a negative refractive surface, such a shape that a negative refractive power becomes progressively stronger toward a marginal lens portion thereof, and the following conditions are satisfied:
2.15xc3x9710xe2x88x923 less than |xcex942(10)/f1| less than 2.45xc3x9710xe2x88x922
1.35xc3x9710xe2x88x923 less than |xcex942(9)/f1| less than 1.60xc3x9710xe2x88x922
4.85xc3x9710xe2x88x924 less than |xcex942(7)/f1| less than 5.60xc3x9710xe2x88x923xe2x80x83xe2x80x83(5)
where xcex942(10), xcex942(9) and xcex942(7) are amounts of asphericity at portions corresponding to 100%, 90% and 70%, respectively, of an effective lens diameter of the aspheric surface (AS2), and f1 is a focal length of the first lens unit.
In accordance with a preferred aspect of the invention, in the above zoom lens, the first lens subunit comprises, in order from the object side, at least two negative lenses and at least one positive lens, the negative lens located on the most object side having a meniscus shape or a plano-concave shape having a strong concave surface facing the image side, and the following condition is satisfied:
xcex94xcexd11nxe2x88x92xcex94xcexd11p greater than 26.5xe2x80x83xe2x80x83(6)
where xcex94xcexd11n is a mean value of Abbe numbers of materials of the at least two negative lenses, and xcex94xcexd11p is an Abbe number of a material of the positive lens.
In accordance with a preferred aspect of the invention, in the above zoom lens, the second lens subunit moves toward the image side during focusing from an infinitely distant object to a minimum-distance object, and comprises at least one positive lens of a shape having a convex surface facing the image side.
In accordance with a preferred aspect of the invention, in the above zoom lens, the third lens subunit comprises at least one negative lens and at least three positive lenses, and the following condition is satisfied:
xcex94xcexd13pxe2x88x92xcex94xcexd13n greater than 37.4xe2x80x83xe2x80x83(7)
where xcex94xcexd13n is an Abbe number of a material of the negative lens, and xcex94xcexd13p is a mean value of Abbe numbers of materials of the at least three positive lenses.
In accordance with a preferred aspect of the invention, in the above zoom lens, the second lens subunit and the third lens subunit respectively have aspheric surfaces.
In accordance with a preferred aspect of the invention, in the above zoom lens, the second lens subunit moves along an optical axis during focusing, an image forming magnification of the second lens unit varies within a range including xe2x88x921xc3x97 during the variation of magnification, and the following conditions are satisfied:
1.2xe2x89xa6|f11/f1|xe2x89xa61.7xe2x80x83xe2x80x83(8)
4.0xe2x89xa6|f12/f1|xe2x89xa67.0xe2x80x83xe2x80x83(9)
1.1 less than |f13/f1|xe2x89xa61.7xe2x80x83xe2x80x83(10)
where f11 is a focal length of the first lens subunit, f12 is a focal length of the second lens subunit, f13 is a focal length of the third lens subunit, and f1 is a focal length of the first lens unit.
In accordance with a preferred aspect of the invention, in the above zoom lens, the second lens subunit has, on at least one lens surface thereof satisfying hW greater than hT where hT is a maximum height of incidence of an on-axial light flux and hW is a maximum height of incidence of an off-axial light flux of a maximum angle of view at a wide-angle end, an aspheric surface having, when formed on a positive refractive surface, such a shape that a positive refractive power becomes progressively stronger toward a marginal lens portion thereof, or an aspheric surface having, when formed on a negative refractive surface, such a shape that a negative refractive power becomes progressively weaker toward a marginal lens portion thereof.
In accordance with a preferred aspect of the invention, in the above zoom lens, the third lens subunit has at least one aspheric surface having, when formed on a positive refractive surface, such a shape that a positive refractive power becomes progressively weaker toward a marginal lens portion thereof, or at least one aspheric surface having, when formed on a negative refractive surface, such a shape that a negative refractive power becomes progressively stronger toward a marginal lens portion thereof.
In accordance with a preferred aspect of the invention, in the above zoom lens, the second lens subunit has one positive lens (12p), the third lens subunit has one negative lens (13n), and the following conditions are satisfied:
N12pxe2x89xa61.67xe2x80x83xe2x80x83(11)
1.76xe2x89xa6N13nxe2x80x83xe2x80x83(12)
xe2x88x920.4xe2x89xa6xcex2VWxe2x89xa6xe2x88x920.2xe2x80x83xe2x80x83(13)
where N12p is a refractive power of a material of the positive lens (12p), N13n is a refractive power of a material of the negative lens (13n), and xcex2VW is a lateral magnification of the second lens unit at a wide-angle end.
In accordance with a preferred aspect of the invention, in the above zoom lens, the first lens subunit comprises, in order from the object side to the image side, at least one negative lens of meniscus form having a convex surface facing the object side, a negative lens of bi-concave form, and a positive lens having a convex surface facing the object side.
In accordance with a preferred aspect of the invention, in the above zoom lens, the second lens subunit comprises a positive lens having a convex surface facing the image side, comprises, in order from the object side to the image side, a positive lens having a convex surface facing the image side and a cemented lens composed of a positive lens and a negative lens, or comprises, in order from the object side to the image side, two positive lenses and a cemented lens composed of a negative lens and a positive lens.
In accordance with a preferred aspect of the invention, in the above zoom lens, the third lens subunit comprises, in order from the object side to the image side, a cemented lens composed of a positive lens and a negative lens, a cemented lens composed of a negative lens and a positive lens and a positive lens, comprises, in order from the object side to the image side, a cemented lens composed of a negative lens and a positive lens and a positive lens, or comprises, in order from the object side to the image side, a cemented lens composed of a negative lens and a positive lens, a positive lens and a positive lens.
In accordance with a preferred aspect of the invention, the above zoom lens is included in a camera system.